The present invention relates to a switching power supply unit.
Heretofore, various types of switching power supply units have been known. One such example includes a phase-shifting full-bridge type switching power supply unit as disclosed in the U.S. Pat. No. 4,864,479.
The phase-shifting full-bridge type switching power supply unit uses four switching elements for a switching circuit. These switching elements are connected to each other in a bridge connection. One pair of connection points originated from the bridge connection forms DC voltage input ends, and the other pair of connection points form switching output ends. A DC power supply is electrically connected to the pair of DC voltage input ends, and a primary winding of a transformer is connected to the pair of switching output ends. An output circuit is connected to the secondary winding of the transformer. A switching output yielded at a secondary winding of the transformer is rectified and smoothed by the output circuit to provide a DC output voltage.
In a switching operation, the four bridge-connected switching elements are driven with the same ON-width and frequency. The four switching elements are on-off controlled by a control circuit to provide a simultaneous ON-period for each of two pairs of the switching elements. According to the switching operation of two sets each composed of either two of the switching elements having the simultaneous ON-period, the direction of a current passing through the primary winding of the transformer is bi-directionally changed to excite the transformer.
In this case, the four switching elements are adequately combined into the two sets to allow the switching elements to be driven in a timing at which the DC voltage input end connected with the DC power source can avoid to be electrically short-circuited in any switching operation.
Moreover, in each set of the two switching elements having the simultaneous ON-period, a switching phase-difference between the two switching elements is controlled according to an output-voltage detection signal or the like to stabilize the output voltage.
Each of the four switching elements includes a pair of main electrodes, and a capacitor is connected between the main electrodes. A resonance circuit is formed by this capacitor and a leakage inductance of the transformer. By using the resulting resonance characteristics, the four switching elements can be operated in a zero-volts switching (hereinafter referred to as xe2x80x9cZVSxe2x80x9d).
On the other hand, in such switching power supply units, an input voltage inputted from the DC power source and/or the level of a load to be connected to the unit can fluctuate. However, the switching power supply unit is required to output a rated output voltage, regardless of any fluctuation in the input voltage and/or the load. The switching power supply unit is also desired to achieve the ZVS so as to reduce a switching loss, regardless of any fluctuation in the input voltage and/or the load. The output of the rated output voltage and the ZVS has a close relationship with the inductance value of the resonance circuit. Thus, one effective means for achieving the ZVS is to adjust the inductance value of the resonance circuit adequately.
However, in the above conventional techniques, the leakage inductance of the transformer is used as the inductor of the resonance circuit. This involves a problem of the difficulty in adjusting the inductance value of the resonance circuit.
It is therefore an object of the present invention to provide a switching power supply unit capable of adjusting the inductance value of a resonance circuit thereof.
It is another object of the present invention to provide a switching power supply unit capable of outputting a rated output voltage regardless of any fluctuation in an input voltage and a load.
It is still another object of the present invention to provide a switching power supply unit capable of serving as a foundation for achieving the ZVS regardless of any fluctuation in an input voltage and a load.
In order to achieve the aforementioned objects, a switching power supply unit according to the present invention comprises a switching circuit, a transformer, an output circuit, and an inductor. The switching circuit includes first, second, third and fourth switching elements, a diode, and a capacitor. Each of the switching elements includes a pair of main electrode. Respective ones of the main electrodes of the first and second switching elements are connected to each other to form a first connection point. Respective ones of the main electrodes of the third and fourth switching elements are connected to each other to form a second connection point. The other main electrode of the first switching element and the other main electrode of the third switching element are connected to each other to form a third connection point. The other main electrode of the second switching element and the other main electrode of the fourth switching element are connected to each other to form a fourth connection point. In each of the switching elements, the third and fourth connection points are led to pared DC voltage input ends, respectively, so as to allow a DC voltage to be applied with a unidirectional polarity between the pair of main electrodes.
In each of the first, second, third and fourth switching elements, the diode is disposed with a reverse polarity between the main electrodes to have a forward direction opposite to the above polarity. The capacitor is disposed between the main electrodes in each of the first, second, third and fourth switching elements.
The transformer includes first and second windings. The first winding is connected between the first and second connection points. The output circuit rectifies and smoothes a switching output yielded at the second winding of the transformer to form an output voltage.
The control circuit controls a switching phase-difference between the first and fourth switching elements or a switching phase-difference between the second and third switching elements to provide a simultaneous ON-period for either two of the switching elements, so as to stabilize the output voltage of the output circuit. The inductor is inserted in a path ranging from the first connection point through the first winding to the second connection point to form a resonance circuit in conjunction with the capacitor. The inductance value of the inductor is arranged to provide the output voltage of the output circuit larger than or equal to a rated value when the input voltage has a minimal value, the output current having a maximum value and the switching phase-difference being zero.
In a switching operation, the bridge-connected first, second, third and fourth switching elements are driven with the same ON-width and frequency. According to the switching operation of two sets each composed of either two of the switching elements having the simultaneous ON-period, the direction of a current passing through the first winding of the transformer is alternately changed to excite the transformer.
The switching output yielded at the second winding of the transformer is rectified and smoothed by the output circuit and is output as a DC output voltage.
The first, second, third and fourth switching elements are combined and driven at a timing so as to prevent the DC voltage input end connected with the DC power source from being electrically short-circuited in any stage of the switching operation. Further, in each set of the two switching elements having the simultaneous ON-period, the switching phase-difference caused between the two switching elements is controlled according to an output-voltage detection signal or the like to stabilize the output voltage.
The switching circuit includes the capacitor which is disposed between the main electrodes in each of the first, second, third and fourth switching elements. Moreover, a switching power supply unit according to the present invention comprises the inductor which is inserted in the path ranging from the first connection point through the first winding to the second connection point. Thus, a switching power supply unit according to the present invention can operate the first, second, third and fourth switching elements with the ZVS by using resonance characteristics of the resonance circuit formed by the capacitor provided in each of the first, second, third and fourth switching elements and the inductance of the inductor.
The inductor forming the resonance circuit is provided independently from the transformer. Thus, the inductance value of the resonance circuit can be readily controlled by adjusting the inductance value of the inductor. The inductance value of the inductor is arranged to provide the output voltage larger than or equal to a rated value of the output circuit when the input voltage has a minimal value, the output current having a maximum value and the switching phase-difference being zero. Therefore, the switching power supply unit of the present invention can output the rated output voltage even if the input voltage goes down to exhibit the minimal input voltage and the output current increases to exhibit the maximum output current. As described above, the inductance value of the resonance circuit can be readily controlled by adjusting the inductance value of the inductor. This also allows the inductance value of the resonance circuit in the switching power supply unit of the present invention to be readily arranged in a certain value for achieving the ZVS at a maximum input voltage and minimum output current.
For the inductor of the switching power supply unit, the present invention discloses a condition of the inductance value for outputting the rated output voltage even at the minimum input voltage and the maximum output current. The present invention also discloses a condition of the inductance value for achieving the ZVS even at the maximum input voltage and the minimum output current.
In the present invention, a variable inductor capable of changing the inductance value in response to the output current may be used as the inductor. According to such a variable inductor, the ZVS operation can be achieved in a wide range of a low load current to a rated load current by appropriately selecting a change characteristic of the inductance value with respect to the change of the output current.
In the present invention, the term xe2x80x9cminimum input voltagexe2x80x9d means an input voltage value at the time when the output voltage cannot be provided at a minimum value in an output voltage range defined in the specification as gradually reducing the input voltage in a full load state. The term xe2x80x9cfull load statexe2x80x9d herein means a state when the output current exhibits a maximum value in an output current range defined in the specification. The term xe2x80x9cmaximum input voltagexe2x80x9d means an input voltage value at the time when the switching elements cannot be operated with the ZVS as gradually increasing the input voltage in the full load state. The term xe2x80x9cmaximum output currentxe2x80x9d means an output current value at the time when the output voltage cannot be maintained at the minimum value in the output voltage range defined in the specification as gradually increasing the output current from the full load state. The term xe2x80x9cminimum output currentxe2x80x9d means an output current value at the time when the switching elements cannot be operated with the ZVS as gradually reducing the output current from the full load state. Further, in the present invention, the term xe2x80x9crated output voltagexe2x80x9d means a maximum value or an equivalent value thereto in the output voltage range defined in the specification of the switching power supply unit.
Another objects, features and advantages of the present invention will be apparent from the following detailed description with reference to the accompanied drawings. The accompanied drawings are simply intended to provide examples of the present invention.